Internal combustion engines are energy conversion mechanisms used by the great majority of motor vehicles and basically comprise two main parts-one or more cylinder heads and the engine block. Below the cylinder head(s) are located the combustion chambers and the cylinders and the crankshaft assembly is located in the engine block.
The crankshaft is a fundamental component in the functioning of combustion engines, and is responsible for converting the energy generated by combustion of the air/fuel mixture into torque. The crankshaft, which is mounted in the block by supporting bearings, is usually of cast carbon steel and has connecting rods coupled to the crank pins and their geometry is reminiscent of a number of cranks placed side by side.
Among the components which help to mount the crankshaft in the engine block are bushes and thrust washers, the thrust washers being responsible for withstanding the axial loads generated by the gearing or gearshift system.
Thrust washers are generally located in one or more bearing positions, and basically comprise two half-moons or two half-washers positioned at the top and bottom of the bearing. These washers act in such a way as to cancel out possible axial forces acting on the crankshaft, so that when it is exposed to an axial load the force generated does not act directly on the engine block because the washer provides a supporting surface for the load, preventing wear on the block.
As may be seen in FIG. 1, a thrust washer is provided with a metal base, substantially of a semi-circumferential shape, and has two surfaces. The contact surface is associated with the engine block, and there is no relative movement between the washer and the block, while the slip surface remains in contact with the crankshaft, giving rise to relative movement between the washer and the crankshaft. Depending upon the applied load, the relative movement places a high load on the contact between the slip surface and the crankshaft, the slip surface being responsible for supporting the axial load produced. In addition to this, thrust washers also have an internal diameter, associated with the shaft, and channels for oil.
The oil channels are fundamental to the proper functioning of thrust washers and ensure that surfaces are lubricated, preventing accelerated/excessive wear on the bearings. These channels are filled with oil so that the oil is distributed over the surface through the pull generated by the rotational movement.
Both Otto and Diesel engines are subject to axial loads. Furthermore, these loads are even greater mainly in engines provided with automatic or automated gear changes. These gears help to reduce fuel consumption in motor vehicles, but they increase the applied loads, with the result that the thrust washers currently in use cannot withstand the forces and become worn or suffer other deficiencies.
In order for the washer to support the loads generated an oil film has to be formed between the slip surface of the washer and the crankshaft. This oil film must maintain a thickness, and it is created through a phenomenon known as hydrodynamic support. This hydrodynamic support depends on two factors—the shape of the surface and the speed.
A washer is said to be hydrodynamically supported when there is no metal-to-metal contact, that is when there is a distance between the surfaces, this distance being the thickness of the oil film. For hydrodynamic support to occur the thickness of the oil film must be three times greater than the surface roughness.
In order to achieve sufficient hydrodynamic support a specific rotation speed, and the best surface possible for that condition, have to be guaranteed. As the rotation speed varies in relation to the speed of the vehicle, the surface can be modified in such a way as to achieve sufficient support to support high axial loads and forces.
In the state of the art there are flat washers, convex washers and ramp-and-pad washers, which basically comprise at least one ramp section, at least one flat horizontal section and at least one channel.
Washers having a flat slip surface, as the name suggests, do not have any geometry on the slip surface, and only include oil channels. These washers have low resistance to wear and are only suitable for engines with low loads.
Convex washers are manufactured by a stamping process and comprise a curvilinear slip surface. They are cheaper, but they have low resistance to wear because of the great variation in the geometry of the slip surface, and because of the lack of flat sections. It should be noted that a flat section is essential in order to maintain hydrodynamic support.
Furthermore ramp-and-pad washers are produced by a machining process and comprise at least one ramp with a constant inclination, at least one horizontal flat surface and at least one oil channel. These washers ensure a high level of hydrodynamic support because the machining process guarantees that the washer is dimensionally stable, and the flat horizontal section ensures a high level of hydrodynamic support, improving the performance of the washers.
Although ramp-and-pad washers have greater resistance to wear and better performance, the connection between the ramp section and the horizontal section represents a sudden change in geometry. Mathematically speaking, this point of connection gives rise to an inflection point or vertex which interrupts the distribution of oil on the slip surface, reducing hydrodynamic support at that point. This inflection point or vertex results in a change in pressure behaviour, giving rise to instability.
One way of reducing this shortcoming is to round off the said vertex linking the ramp section to the flat horizontal section by means of a small radius. This configuration results in less instability in the washer, but it does not ensure excellent performance.
In view of the above, there has not yet been developed any thrust washer which has a surface which guarantees uniform hydrodynamic support which is not affected by any transition effect, imparting high wear resistance to the washer and consequently excellent performance.